Low noise hybridized detector using charge transfer

ABSTRACT

A low noise infrared photodetector has an epitaxial heterostructure that includes a photodiode and a transistor. The photodiode includes a high sensitivity narrow bandgap photodetector layer of first conductivity type, and a collection well of second conductivity type in contact with the photodetector layer. The transistor includes the collection well, a transfer well of second conductivity type that is spaced from the collection well and the photodetector layer, and a region of first conductivity type between the collection and transfer wells. The collection well and the transfer well are of different depths, and are formed by a single diffusion.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Application No.62/032,918 filed Aug. 4, 2014 for “LOW NOISE HYBRIDIZED DETECTOR USINGCHARGE TRANSFER” by P. Dixon and N. Masaun.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under Contract No.N00014-12-C-0375 awarded by Office of Naval Research. The government hascertain rights in the invention.

BACKGROUND

This invention generally relates to a device for detecting radiation inthe near infrared (IR) spectrum. In particular, the invention relates toa low noise IR detector that operates by transferring charge rather thanby charging and resetting a capacitor through which voltage is read.

Modern infrared (IR) imaging systems can be focal plane arrays ofdetectors and associated integrated circuitry in each pixel thattransforms the collected signals into visual or other analyzable forms.Near IR detector systems that operate in the 1 to 1.7 μm wavelengthregion are sometimes combined with visible detection systems thatoperate in the 400 to 700 nm wavelength range to enhance detection andvisualization in low light and early night scenarios. Combined visibleand near IR imaging capability is increasingly becoming a strategicrequirement for both commercial and military applications. Of the manymaterials used for imaging systems that operate in the near infrared(e.g. HgCdTe, Ge, InSb, PtSi, etc.), InGaAs p-i-n photodiodes have beenchosen due to their high performance and reliability (G. Olsen, et al.,“A 128×128 InGaAs detector array for 1.0-1.7 microns,” in ProceedingsSPIE, Vol. 1341, 1990, pp. 432-437).

Short wavelength infrared (SWIR) imaging arrays are normally hybriddevices where the photodiodes are interconnected to silicon transistorread out integrated circuitry (ROIC). In one effort to decrease cost andsimplify complex manufacturing, an InGaAs/InP photodiode has beenintegrated with an InP junction field effect transistor (JFET) as aswitching element for each pixel, as described by U.S. Pat. No.6,005,266, Forrest et al. (which is incorporated herein by reference inits entirety). The combination of photodiode and FET on a singlesubstrate enabled the formation of fully monolithic near IR focal planearrays with reduced production cost and increased performance. The InPjunction field effect transistors exhibited leakage currents as low as 2pA. In related work, intentional doping of the absorption layer of aGaAs p-i-n photodiode was found to reduce the dark current as describedby U.S. Pat. No. 6,573,581, Sugg. et al. (which is incorporated hereinby reference in its entirety).

In previous detectors, light induced charge is collected in a singlearea that is then transferred to an external capacitor where the voltageon the capacitor is measured. The capacitor is then “reset” before thenext measurement. Since it is difficult to completely reset a capacitorin a finite amount of time, and the collection area may be collectingcharge during the reading operation itself, opportunities exist forvariation in the amount of signal read.

SUMMARY

An infrared photodetector includes a small bandgap layer of firstconductivity type; a large bandgap layer of first conductivity typeoverlaying the small bandgap layer; a collection well of secondconductivity type in the large bandgap layer and in contact with thesmall bandgap layer so that the small bandgap layer and the collectionwell form an infrared photodiode; a standoff layer over a portion of thelarge bandgap layer; a transfer well of second conductivity type in thestandoff layer and the large bandgap layer and spaced from thecollection well and the small bandgap layer; and a transistor thatincludes the collection well, the transfer well and a region between thecollection well and the transfer well.

In another embodiment, an infrared photodetector includes a smallbandgap layer of a first conductivity type, a large bandgap layer of afirst conductivity type on the small bandgap layer, and a second smallbandgap layer on a portion of the second large bandgap layer. A transferwell of a second conductivity type is located in the second smallbandgap layer and large bandgap layer. The transfer well is separatedlaterally from the collection well and vertically from the first smallbandgap layer. Electrodes are positioned to cause charge transfer fromthe collection well to the transfer well.

In a further embodiment, a method of forming an infrared photodetectorincludes depositing a large bandgap layer of a first conductivity typeon a small bandgap layer of a first conductivity type, and forming astandoff layer on a portion of the large bandgap layer. In a singlediffusion step, a collection well of a second conductivity type isformed in large bandgap layer, and the first small bandgap layer, and atransfer well of a second conductivity type is formed in the standofflayer and the large bandgap layer. The transfer is spaced laterally fromthe collection well and vertically from the first small bandgap layer.Electrodes on the small bandgap layer are formed to allow chargetransfer from the collection well to the transfer well.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a photodetector/transistor devicearchitecture of the invention.

FIGS. 2A-2C are illustrations showing the operation of the photodetectorof the invention.

FIG. 3 is a schematic illustration of the photodetector and related readout integrated circuitry (ROIC) of the invention.

FIGS. 4A-4G are schematic illustrations of the formation steps of theinvention.

FIGS. 5A and 5B show alternate versions of the photodetector/transistordevice.

DETAILED DESCRIPTION

FIG. 1 shows device 10, which includes integrated short wavelengthinfrared photodetector PD and low noise, epitaxial, multi-layer fieldeffect transistor T1. This device architecture uses one area for chargecollection and a separate area for charge measurement. In addition, acapacitor is not needed for measurement of signal level.

Although device 10 will be described based on InGaAs/InP material anddevice technology, the methods and features discussed herein are notintended to be limited to that material system alone, and othersemiconductor materials, including other III-V and II-VI compoundsemiconductor materials, are included in the scope of the invention.

Device 10 is a multilayer structure including n type large bandgap baseor substrate layer 12, n type small bandgap photosensor layer 14, n typelarge bandgap layer 16, small bandgap standoff layer 18, p typecollection well 20, p type transfer well 22, source contact 24, gatetransfer contact 26, and drain contact 28, and insulator layer 30. Ntype layer 14 and collection well 20 form short wavelength infrared(SWIR) photodiode PD. Layers 14, 16 and 18, collection well 20, transferwell 22, source electrode 24, gate transfer electrode 26 and drainelectrode 28 form lateral junction field effect transistor (JFET) T1.

In one embodiment, n type large bandgap substrate layer 12 is InP with abandgap of about 1.344 eV. N type small bandgap layer 14 is InGaAs witha thickness of about 3 to 3.5 μm and bandgap of about 0.74 eV. N typelarge bandgap layer 16 is InP with a thickness of about 0.5 to 1 μm anda bandgap of about 1.344 eV. Small bandgap standoff layer 18 is InGaAswith a thickness of about 0.5 μm or less and a bandgap of about 0.74 eV.

In this embodiment, P type collection well 20 is formed by diffusioninto InP layer 16 and InGaAs layer 14. As a result, collection well 20has a two layer structure comprising layers 20A and 20B. Layer 20A ofcollection well 20 is InP with a thickness of about 0.5 to 1.0 μm. Layer20B of collection well 20 is InGaAs with a thickness of about 0.25 μm.Transfer well 22 is formed by diffusion into standoff layer 18 and largebandgap layer 16. As a result, transfer well 22 has a two-layerstructure comprising layers 22A and 22B. Layer 22A of transfer well 22is InGaAs with a thickness of about 0.5 μm or less. Layer 22B oftransfer well 44 is InP with a thickness of about 0.05 μm.

Source electrode 24, gate transfer electrode 26, and drain electrode 28may be Au, Cu, Ag, Pd, Pt, Ni and others known in the art.

Schematic figures illustrating the operation of device 30 are shown inFIGS. 2A-2C. The device operates by collecting photoinduced carriers inP type collection well 20 of photodiode T1. The collected charge istransferred by transistor T1 from collection well 20 (which acts as thesource region of T1) to transfer well 22 (which acts as the drain regionof T1). The charge in transfer well 22 can then be read out withoutaffecting generation and collection of carriers by photodiode PD.

In FIG. 2A, SWIR radiation is absorbed in high sensitivity photodetectorlayer 14 and produces photo-induced carriers c. In FIGS. 2A and 2B, thecarriers are driven toward collection well 20 as indicated by arrows a,and are swept across the pn junction formed by n type photodetectionlayer 14 and collection well 20. As shown in FIGS. 2B and 2C thecarriers, in collection well 20 are transferred to transfer well 22 asschematically shown by arrow d. A positive voltage on transfer gateelectrode 26 inverts to p-type the underlying large bandgap layer 16between collection well 20 and transfer well 22. This allows charges cin collection well 20 to move to transfer well 22. The charges intransfer well 22 may then be sampled by an external ROIC circuit. Thereis complete charge transfer and no reset noise is generated during thetransfer.

FIG. 3 shows device 10 with a portion of the ROIC circuitry. TransistorT1 of device 10 forms one transistor of a five transistor (5T)architecture used by the ROIC to acquire the photosignal generated byphotodiode PD of device 10 for measurement. In a SWIR array, there willbe an array of devices 30, together with associated 5T circuits. Thephotosignals are delivered by the 5T circuits to measurement and furthersignal processing circuitry (not shown).

The 5T circuit in FIG. 3 includes field effect transistors T1-T5 andoptional capacitor C1. Transistor T2 is a reset transistor that isturned on to reset device 10 for the next charge transfer and readoutcycle by connecting transfer well 22 to ground. This resets transferwell 22 before the next transfer of carriers from collection well 20.

Transistor T3 has its gate connected to drain contact 28 of device 10.Transistor T3 acts as a source follower, with its source voltage being afunction of the charge in transfer well 22.

Transistors T4 and T5 are sample select and column select switches,respectively, that select the photosignal being delivered to the furtherROIC circuitry. Capacitor C1 is used if it is desired to perform sampleand column selects sequentially rather than simultaneously. In thatcase, voltage at the source of T3 is stored in capacitor C1 and thenread out by turning on column select transistor T5.

A method of forming device 10 is shown in FIGS. 4A-4G, in whichcollection well 20 and transfer well 22 are formed with a singlediffusion. Standoff layer 18 allows transfer well 22 to be shallowerthan collection well 20. As a result, p-type collection well 20 is incontact with n-type small bandgap layer 14 to form photodetector PD.P-type transfer well 22, on the other hand, is located only in standofflayer 18 and large bandgap layer 16. The pn junction formed by transferwell 22 and large bandgap layer 16 is not responsive to SWIR radiation.

The starting material shown in FIG. 4A is a multilayer heterostructuremade up of layers 12, 14, and 16. As an example, layers 12 and 16 may beInP, and layer 14 may be InGaAs. The compositions and thicknesses of onespecific embodiment have already been described. The heterostructure maybe formed by any epitaxial growth process known in the art. Examplesinclude organometallic vapor phase epitaxy (OMVPE), metal organicchemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), andothers known in the art. A preferred technique is MOCVD. In otherembodiments, layer 14 may also be the substrate of the device, and layer12 is not present.

In the next step, as shown in FIG. 4B, standoff layer 18 is deposited onlayer 16. In FIG. 4C, portions of standoff layer 18 have been removed.This can be achieved using photoresist masking and etching, or by otherselective removal techniques.

As shown in FIG. 4D insulator layer 30 has been deposited over the topsurface of layer 16 and over standoff layer 18. Insulator layer 30 is,for example, silicon nitride or silicon oxynitride. Insulator layer 30is an electrical insulator, and also functions as a diffusion barrier.

In FIG. 4E, diffusion windows 40 and 42 have been formed in insulatorlayer 30. Window 40 defines an opening through which a p-type dopant canbe introduced for collection well 20. Window 42 defines an openingthrough which the p-type dopant can be introduced to form transfer well22.

In FIG. 4F, a diffusion step has been performed to form collection well20 and transfer well 22. In the case of collection well 20, the p-typedopant diffuses through large bandgap layer 16 into the upper portion ofsmall bandgap layer 14. In the case of transfer well 22, the p-typedopant must first diffuse through standoff layer 18 before reachinglarge bandgap layer 16. The diffusion rate of the p-type dopant is lowerin standoff layer 18 than in large bandgap layer 16. As a result,transfer well 22 is confined to standoff layer 18 and the upper portionof large bandgap 16. In this way, collection well 20 and transfer well22 are formed with a single diffusion, yet have different depths.

In the final step, as shown in FIG. 4G, contact regions are defined byphotolithography and source contact 24, gate transfer contact 26, anddrain contact 28 are deposited on collection well 20, insulator layer30, and standoff layer 18, respectively. Contacts 24, 26, and 28 aredeposited by photolithography, sputtering, electroplating or other meansof deposition known in the art. Preferred contact materials are Au, Cu,Ag, Pd, Pt, Ni and others known in the art. In some embodiments, sourcecontact 24 is not needed, and can be omitted. Standoff layer 18 is asmall bandgap semiconductor, such as InGaAs, which helps to form anohmic contact at drain contact 28.

Alternate versions 10A and 10D of the photodetector/transistor deviceare illustrated in FIGS. 5A and 5B, respectively. In device 10A of FIG.5, collection well 20 forms a ring around transfer well 22 and gatecontact 26. Note that source contact is not present in device 10A.

In FIG. 5B, device 10B also uses a ring configuration. In thisembodiment, transfer well 22 forms a ring around collection well 20 andgate contact 26.

The photodetector/transistor structures shown in FIGS. 1-2B, 5A and 5Boffer certain design and manufacturing features and benefits. Theyinclude:

Charge well 42 may be a buried p type diffusion layer surrounded by adetailed bandgap engineered material on all sides except through thecharge collection region. This allows charge to be collected whilekeeping collected dark current low and separating the collection areafrom the surface of the InGaAs material. This buried layer minimizesboth surface recombination and shunt contribution to noise.

The integrated photodetector/transistor structure, plus a mechanism fordumping the charge from transfer well 22 may be included in 5-8micrometer pixels.

By avoiding the capacitor reset noise of the prior art, the inherentnoise of the pixel may be orders of magnitudes lower than prior artdevices.

The architecture of the present invention may achieve up to about fivetimes (5×) greater sensitivity enabling night imaging at below starlightlevels while reducing pixel pitch by up to about three times (3×). As aresult, the detectors may operate at lower light conditions; operate athigher operating temperature for a given light level; operate at lowerpower since, for example, cooling is not needed to improve performance;and higher resolution is achieved in a smaller detector with smalleroptics and a higher areal density of chips on a wafer, resulting inreduced cost.

The integrated structure can be formed using only two differentsemiconductor materials (such as InP and InGaAs) and a single diffusionto achieve collection and transfer wells of different depths.

Discussion of Possible Embodiments

The following are nonexclusive descriptions of possible embodiments ofthe present invention.

An infrared photodetector includes a small bandgap layer of firstconductivity type; a large bandgap layer of first conductivity typeoverlaying the small bandgap layer; a standoff layer on a portion of thelarge bandgap layer; a collection well of second conductivity type inthe large bandgap layer and in contact with the small bandgap layer sothat the small bandgap layer and the collection well form an infraredphotodiode; a transfer well of second conductivity type in the standofflayer and the large bandgap layer and spaced from the collection welland the small bandgap layer; and a transistor that includes thecollection well, the transfer well and a region between the collectionwell and the transfer well.

An infrared photodetector may include a first small bandgap layer of afirst conductivity type; a large bandgap layer of a first conductivitytype may be on the first bandgap layer; a second small bandgap layer ona portion of the large bandgap layer; a collection well of a secondconductivity type may be located in the first small bandgap layer andlarge bandgap layer; a transfer well of a second conductivity type maybe located in the second small bandgap layer and the first large bandgaplayer, the transfer well being spaced laterally from the collection welland vertically from the first small bandgap layer; and electrodes on thesecond small band gap layer may be positioned to cause transfer ofcharge from the collection well to the transfer well.

A drain electrode coupled to the transfer well; and a gate electrode iscoupled to the region between the collection well and the transfer well.

The gate and drain electrodes comprise Ti, Pt, Au, Ni, Cu, orcombinations thereof.

An insulator layer is located between the gate electrode and the largebandgap layer.

The transfer well extends to a top surface of the standoff layer.

The transfer well extends to a top surface of the second small bandgaplayer.

The collection well extends to a top surface of the large bandgap layer.

The standoff layer comprises a small bandgap semiconductor.

The small bandgap layer and the standoff layer comprise InGaAs, and thelarge bandgap layer comprises InP.

The collection well extends to a top surface of the large bandgap layer.

The transfer well extends to a top surface of the second small bandgaplayer.

A method of forming an infrared photodetector may include: depositing asmall bandgap layer of first conductivity type; depositing a largebandgap layer of first conductivity type on the first small bandgaplayer of first conductivity type; forming a standoff layer on a portionof the large bandgap layer; and forming by a single diffusion acollection well of second conductivity type located in the large bandgaplayer and the small bandgap layer, and a transfer well of secondconductivity type located in the standoff layer and the large bandgaplayer and spaced from the collection well and the small bandgap layer.

The method of the preceding paragraph can optionally include,additionally and/or alternatively any, one or more of the followingfeatures, configurations, and/or additional components:

The method comprises forming electrodes overlying the collection well,the transfer well, and a region between the collection well and thetransfer well.

The standoff layer comprises depositing a second small bandgap layer onthe large bandgap layer; and selectively removing a portion of thesecond small bandgap layer to define the standoff layer.

Forming by a single diffusion step comprises depositing an insulatorlayer over the large bandgap layer and the standoff layer; forming anopening in the insulator layer over a portion of the large bandgap layerto define a location of the collection well; forming an opening in theinsulator layer over the standoff layer to define a location of thetransfer well; and diffusing a dopant through the openings in theinsulator layer to form the collection well and the transfer well.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

The invention claimed is:
 1. An infrared photodetector comprising: asmall bandgap layer of first conductivity type; a large bandgap layer offirst conductivity type overlying the small bandgap layer; a standofflayer on a portion of the large bandgap layer; a collection well ofsecond conductivity type in the large bandgap layer and in contact withthe small bandgap layer so that the small bandgap layer and thecollection well form an infrared photodiode; a transfer well of secondconductivity type in the standoff layer and the large bandgap layer andspaced from the collection well and the small bandgap layer; and atransistor that includes the collection well, the transfer well and aregion between the collection well and the transfer well.
 2. Theinfrared photodetector of claim 1, wherein the transistor furtherincludes: a drain electrode coupled to the transfer well; and a gateelectrode coupled to the region between the collection well and thetransfer well.
 3. The infrared photodetector of claim 2, wherein thegate and drain electrodes comprise Ti, Pt, Au, Ni, Cu, or combinationsthereof.
 4. The infrared photodetector of claim 2, and furthercomprising: an insulator layer between the gate electrode and the largebandgap layer.
 5. The infrared photodetector of claim 1 wherein thetransfer well extends to a top surface of the standoff layer.
 6. Theinfrared photodetector of claim 1 wherein the collection well extends toa top surface of the large bandgap layer.
 7. The infrared photodetectorof claim 1, wherein the standoff layer comprises a small bandgapsemiconductor.
 8. The infrared photodetector of claim 1, wherein thesmall bandgap layer and the standoff layer comprise InGaAs, and thelarge bandgap layer comprises InP.
 9. An infrared photodetectorcomprising: a first small bandgap layer of first conductivity type; alarge bandgap layer of first conductivity type on the first smallbandgap layer of first conductivity type; a second small bandgap layeron a portion of the large bandgap layer; a collection well of secondconductivity type located in the first small bandgap layer and the largebandgap layer; a transfer well of second conductivity type located inthe second small bandgap layer and the first large bandgap layer, thetransfer well being spaced laterally from the collection well andvertically from the first small bandgap layer; and electrodes positionedto cause transfer of charge from the collection well to the transferwell.
 10. The infrared photodetector of claim 9, wherein the collectionwell extends to a top surface of the large bandgap layer.
 11. Theinfrared photodetector of claim 9, wherein the transfer well extends toa top surface of the second small bandgap layer.